Generally, in optical recording, the same beam which is used for reading information from tracks on optical media is also used for recording information on the same media. Where such is the case, two distinctly different power levels are used for reading and writing operations. Usually a very low power reading beam is employed to attenuate concerns over the potential problems involved with reading and re-reading an area in a short period of time, which, if the read beam is strong enough, could result in damage to the disk media surface and loss of recorded information. Depending on the type of media employed such damage could result from any number of potential causes including, unwanted ablation due to successive heating, or other types of changes in the physical structure of the recording substrate or even changes in the protective surface or carrier layers of the recording media. Therefore it is important, especially to applications which may require reading and re-reading of the same track areas, that the read power be very low. A higher power is used to "write" on the media, that is, to cause detectable modulations in its information bearing layer(s).
In the implementation of many optical disk storage systems to which this invention is primarily directed, a direct read during write operation is used to ensure that the data has been written correctly. That is, the data written is read immediately to verify it.
In an implementation employing media such as that employed in the preferred embodiment reflective surface is removed during writing causing ablation in the media surface and resulting in a nonreflective pit in the information bearing layer.